546 SUPPLEMENT ON ECHINODERMATA. 
terized. Some small generic groups have been introduced 
by some naturalists. Thus M. de Lamarck has separated 
the species whose tentacula are formed by small denticulated 
and pedicled disks to form his genus fistularia. M. Oken 
had already extended these divisions much further, since he 
has established four genera according to the consideration of 
the disposition of the retractile tubes, and the form of the 
body. As these animals are extremely altered in appearance, 
when they have been kept for some time in alcohol, it becomes 
rather difficult to characterize the species. They seem to form 
a passage from the radiata to the worms, and perhaps even to 
the mollusea; for, in fact, we find some species which have 
altogether the form of the thalassema, and others which exhibit 
at the first glance, a resemblance to some dorides. 
